1. Field of the Invention
The present invention relates to a drug delivery system comprising
(a) a reservoir from which set doses are apportioned, PA1 (b) a dose setting mechanism by which the relative position of two elements is changed by a distance proportional with a set dose, and PA1 (c) a push button by activation of which an element is moved a distance corresponding to said distance proportional to the set dose.
2. Description of the Related Art
Mainly the dose setting mechanism comprises dose setting elements which are during the setting of a dose displaced relative to each other in such a way that the pressing of the button until it abuts a stop results in a movement of a piston into one end of an cylinder ampoule a distance corresponding to the set dose whereby the dose is pressed out through a needle mounted at the other end of the ampoule.
An example of such an injection device is described in EP 327 910 wherein during the dose setting a nut mounted on a threaded piston rod is screwed along this piston rod away from a stop, and a button mounted on the nut is lifted up over a proximal end of the housing of the device. When the button is pressed to bring the nut back to abutment with the stop, the piston rod is forced into an ampoule a distance corresponding to the set dose.
Such injection devices have shown to be advantageous as the reservoir may content sufficient medicine for say one week and the handling of vials and disposable syringes or syringes which has to be cleaned after each use is avoided. The reservoir even though it is used for several injections has the advantage of a disposable device as the piston is only once moved all the way from one end of the ampoule to the other whereafter the ampoule is disposed of.
One of the draw-backs of the injection devices of the above mentioned type is the fact that to accommodate the ampoule and a piston rod which shall be able to force the piston from one end of the ampoule to the other, the device has to have a length which is more than twice the length of the ampoule. This drawback may however be overcome by using a flexible piston rod which may be bent away from the axis of the ampoule immediately behind this ampoule.
Another draw-back is that the piston area is made larger and consequently the movement of the piston have to be smaller to inject a given dose when the ampoule is given a larger diameter to have a larger volume. This again makes it more tricky to perform a precise setting of a dose and inaccuracies due to the elasticity of the piston become more probable. Further the pressure which has to be exerted on the piston rod increases proportionally with the piston area at the same time as a trend toward the use of thinner needles calls for still higher pressures in the injection cylinder formed by the ampoule. Also a wish for the development of injection devices by which the medicine is pressed out as a skin penetrating jet points towards the use of higher pressures in the injection cylinder. This trend was not foreseen by the development of the mainly used ampoule which consequently is not guaranteed to withstand excessive inner pressures.
An object of the invention is to provide an injection device, by which the above mentioned draw-backs are overcome.